1. Field of the Invention
The present invention relates to a zoom lens system for photographic cameras, and more specifically to a zoom lens system suited for use with photographic cameras which use solid-state image pickup devices as image sensors.
2. Description of the Prior Art
Most of the lens systems which are adopted for use with cameras using electronic image pickup tubes or solid-state image pickup devices as image sensors are designed as zoom lens systems, as exemplified by the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 62-247318 and No. Sho 63-287810. Each of these zoom lens systems comprises, in the order from the object side, a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth positive lens unit, and adapted to move the second lens unit and the third lens unit for variation of focal length. These zoom lens systems have compositions consisting of small numbers of lens elements, compact external designs and high zooming ratios exceeding 3. Especially, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-287810 has a high aperture ratio exceeding F/2.8, and a long back focal length whose optical path length is 1.05 times as long as a geometrical mean f.sub.s of the focal lengths of the zoom lens system as a whole at the wide position and that at the tele position.
Further, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-236514 has a long back focal length whose optical path length f.sub.B is longer than 1.2 times of the focal length f.sub.s for allowing to arrange a quick return mirror which is used to branch an optical path of a finder system from the optical path of the photographic system, a composition consisting of a small number of lens elements, a field angle of approximately 48.degree. at the wide position, a vari-focal ratio of approximately 3, an aperture ratio of approximately f/2.8, a total length of approximately 4.5 f.sub.s, an effective diameter of a front lens component of approximately 1.55 f.sub.s. That is to say, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-236514 is compact in external design thereof, light in weight, manufacturable at a low cost and high in optical performance.
However, the conventional examples of zoom lens systems mentioned above have the defects described below. The zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-287810 uses a thick positive lens component in a fourth lens unit comprised therein and has a long total length.
Further, each of the lens systems disclosed by Japanese Patent Kokai Publication No. Sho 62-247318 and Japanese Patent Kokai Publication No. Hei 2-236514 uses a thick negative lens component and has a long total length.
All of the conventional examples of zoom lens systems described above are long in the total lengths thereof and low in portability.